After doing some research, and calculating my needs, I decided to set up an inverter based standby power supply for when Eskom starts again with their power cuts. I also planned the setup to be able to accept solar panels in the future, if/when the national electricity supply becomes so bad that there is no reliable electricity supply to re-charge the standby batteries. Finally, I designed the system to provide a long "operating life" with the batteries lasting for at least 5 years (and hopefully more than 10 years) before I need to replace them.

My "critical" electrical requirements are as follow:

All the tank's circulation pumps, including the sump return pump and skimmer pumps

Lights throughout the house (but not all burning at the same time)

Supply to the deep fridge

Supply to the computer in my study

Some spare capacity to be able to run either a kettle or the microwave oven if I turn off most of the lights

Based on the above, I determined that I would need at least a 2.0kVa inverter, with a short-term capacity of at least 3.0 kVa to handle the inrush current when pumps start up, etc. After a good deal of searching and getting quotations, I settled on the Studer XPC 2200-24 Compact inverter. This unit is made in Switzerland, is a true sine wave inverter, can conservatively supply 2.2kVa and has a 5 second "inrush" capacity of 6.6kVa. Although more expensive than the typical Chinese imports, it is ruggedly built, has a decent built-in battery charger, and comes highly recommended by some of the "boffins" in the industry.

Early in my research, I discovered that an inverter is only as good as the batteries supplying it with DC power. I wanted a system that would be able to give me a reasonably long back-up supply, and not cause the batteries to fail prematurely by discharging them too deep (or not charging them sufficiently). I thus calculated my requirements as follow:

Power requirement = 2kVa = 2000W @ 230V

Assuming an inverter efficiency of 90% (the Studer is rated at 95%), and given that it is a 24V unit, I thus need an input current of 2000W / 0.9 / 24V = 92.6A @ 24V.

After studying various makes and types of batteries' data sheets, I settled on using the TPL 121600 batteries from CSB (a well known, reputable Chinese manufacturer). These batteries are rated at 160Ah, and has a nominal "stand-by" life of 10 years. The data sheet of this battery indicates that a discharge down to 1.8V/cell (10.8V) can supply 44.1A for a period of 3 hours. Using two batteries in parallel would thus give me a capacity of 88.2A for three hours, or 96.6A for two hours and 45 minutes. With a combination of two batteries in series (to give me 24 Volt) and two batteries in parallel (to double the Amperage) I can thus run the inverter for slightly less than 3 hours at full load.

The inverter's built-in charger supplies up to 37A of charging current at 24 Volt, which is also within a safe range to recharge all four batteries at the same time without over charging them.

OK, enough info - here are some photos:

The inverter:

The batteries:

Notice the thickness of the connecting cables, to be able to handle nearly 100 Amperes:

Yup. One of the most important ways of extending battery life is to keep the battery temperature at or below 25°C, and the thermometer is there to monitor this. My garage is nice and cool year round (being on the south western end of the house), and since I started installing the system in December it has not gone above this temperature. The batteries will heat up when discharging, and it they become too hot I will have to put a fan on them, I suppose.

seems like it's a bit cold there in bloem...

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Cool, yes - cold, not yet... We regularly go down to -5°C in winter, with the odd cold spell taking it down to perhaps -7 or -8

awesome Hennie, hope people have taken note how its to be done properly, and not using a R500 special and twin flex.

just a question Hennie, since you have two sets of batteries in parallel, how are you going to ensure the sets charge equally, is there a "balancing" circuit?Posted via Mobile Device

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definately the proper way hey lighty!

IMO deep cycle batteries seem to balance themselfs out. the fuller the one battery gets, the less current it uses so more current goes to the other one and so one. they probably will be a few millivolts out but thats ok. not like lithium ion battery packs that actually get damaged if the one discharges or charges at a different rate.

since you have two sets of batteries in parallel, how are you going to ensure the sets charge equally, is there a "balancing" circuit?

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Good question, and a very important point!

If you look closely at the second photo, you will see that the two batteries on either side are connected with a solid bus-bar between one's negative terminal to the other's positive terminal. This is the series connection, giving me 24 volt. This 24 volt "battery" is then connected in parallel (positive to positive and negative to negative) with the thick red and black cables. Taking the power from the two ends thus ensures that all four batteries are discharged equally. If one hooks up a third set of batteries this connection becomes more complicated, and one must ensure that all the negative terminals are connected to a single "tab" with the connecting wires of exactly the same length (and the positive terminals likewise...), else the unequal resistance of the different lengths of connecting wires will result in uneven discharging and charging of the various batteries. This becomes an even greater problem if more batteries are used.

IMO deep cycle batteries seem to balance themselfs out. the fuller the one battery gets, the less current it uses so more current goes to the other one and so one. they probably will be a few millivolts out but thats ok. not like lithium ion battery packs that actually get damaged if the one discharges or charges at a different rate.

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Actually, it is pretty important. Although the voltage drop might be only a few millivolt, the problem is that during discharge the current is close to 100 amperes. This makes quite a difference, and if one battery is discharged deeper than the others, it won't be fully charged again because firstly it will need more charge, and secondly the additional resistance in the longer wiring will reduce the charging current somewhat. A partially charged battery will degrade very quickly, and will lose a large portion of it's life in a very short time. Check out the last link which I've posted above...

Well, the inverter cost me R16 100.00, and the batteries were in the vicinity of R4000.00 each, if I remember correctly (can't put my hands on the invoice right now...). I had to install new ducting and wiring for the "back-up" plug points in my study (for the computer), dining room (for the deep freezer) and sitting room (for the tank). I also had to re-route the lighting circuits to run through the inverter.

While I was busy with that, I decided to just carry on and re-do all the wiring in the house, as the old wiring was still running open above the ceiling and were in a pretty bad shape. The wiring certainly did not comply with the new SANS regulations, and the house did not even have an earth leakage unit in the DB board. I thus replaced all the circuit breakers with new ones as well, and installed a separate "sub-board" with it's own earth leakage for the backup supply. I'm not sure how much this all cost me, as the work was done over a period of a few months, but I would guess it put me back another R5 000.00 or there about.

So, all in all, about R37 000.00 Wonder if I can deduct some of that off my taxes, as support for Eskom

Well, the inverter cost me R16 100.00, and the batteries were in the vicinity of R4000.00 each, if I remember correctly (can't put my hands on the invoice right now...). I had to install new ducting and wiring for the "back-up" plug points in my study (for the computer), dining room (for the deep freezer) and sitting room (for the tank). I also had to re-route the lighting circuits to run through the inverter.

While I was busy with that, I decided to just carry on and re-do all the wiring in the house, as the old wiring was still running open above the ceiling and were in a pretty bad shape. The wiring certainly did not comply with the new SANS regulations, and the house did not even have an earth leakage unit in the DB board. I thus replaced all the circuit breakers with new ones as well, and installed a separate "sub-board" with it's own earth leakage for the backup supply. I'm not sure how much this all cost me, as the work was done over a period of a few months, but I would guess it put me back another R5 000.00 or there about.

So, all in all, about R37 000.00 Wonder if I can deduct some of that off my taxes, as support for Eskom